Orthogonal Frequency Division Multiplexing Modem Research Articles

Research and Development of a Highly Reconfigurable OFDM MODEM for Shallow Water Acoustic Communication

Orthogonal frequency division multiplexing (OFDM) provides an effective approach to combat multipath with high bandwidth efficiency for underwater acoustic communication, thus design and implementation of acoustic OFDM MODEMs draw more and more attention. Extensive work has been performed to mitigate multipath, Doppler shift, and peak average power ratio from the viewpoint of theoretical analysis. Meanwhile, design and implementation of a parameter-configurable OFDM to adapt to highly diverse underwater channels and various missions become more and more important. Traditional MODEM with fixed parameters cannot achieve the best tradeoff between data rate and bit error rate. In order to obtain the best communication performance under long time delay channels, Doppler channels, and frequency-selective channels, in this paper, we provide the design and implementation of a parameter-configurable underwater acoustic OFDM MODEM with the purpose to achieve flexibility. Specifically, the reconfigurable parameters of the OFDM MODEM include the number of receivers, the number of null subcarriers, the number of pilot subcarriers, as well as the type of channel estimation algorithms. The number of pilot subcarriers and type of channel estimation methods are determined based on channel characteristics, the number of null subcarriers is determined based on the Doppler shift, and the number of the received channels is based on fading channel. Moreover, in this paper, the Doppler estimation performance under different number of null subcarriers, and the channel estimation performance under different number of pilot subcarriers and under different channel estimation algorithms, are analyzed in terms of bit error rate. Finally, the effectiveness of the proposed highly configurable OFDM MODEM is demonstrated by two sea trials corresponding to different communication scenarios.

Partial discharge detection using PLC receivers in MV cables: A theoretical framework

Partial discharge (PD) activity is a key indicator of the cable insulation health in medium-voltage networks. Many of these cables are used as transmission media, because power line communication (PLC) modems are installed at their ends.This paper proposes two techniques that allow broadband PLC receivers to detect PDs at the same time that the PLC signal is being demodulated. The proposed techniques are based on how PDs alter both, null, and zero-bit loaded carriers, commonly used in orthogonal frequency division multiplexing modems. Using the proposed techniques, a simple firmware modification is sufficient to add this new functionality to the already installed modems.This paper is mainly concerned with theoretical aspects of the proposed techniques, and no experimental validation has been done. Nevertheless, simulation results show that the proposed techniques can be used to detect different types of PDs. Four cases studies are analyzed: short- and long-distance PDs, and low- and high-energy PDs. Finally, the impact of the inter-carrier interference on the detection capability is studied, and some recommendations are made to mitigate its effects.

Performance evaluation and implementation of convolution coded OFDM modem in wireless underwater acoustic communication

SummaryHigh‐data transmission rate and reliable communication in underwater acoustic channel is challenging because of limited bandwidth, multipath propagation, and Doppler shift, which results in poor bit error performance. Under this constraint, this paper explains the simulation results of underwater wireless acoustic data transmission system by using quadrature phase shift keying modulation with convolution encoder at the transmitter and proposed Viterbi decoder at the receiver. The decoder algorithm in comparison with MATLAB inbuilt function shows asserting improved results. This paper evaluates the performance of convolution coded orthogonal frequency division multiplexing modem, which is studied over typical underwater channel through an extensive computer simulation and a semirealistic experimentation. The performance of convolution coded orthogonal frequency division multiplexing system is measured in time domain plots, bit error rate curves, and number of bit errors per frame over additive white Gaussian noise and Rayleigh channel conditions.

Cost-Efficient Frequency-Domain MIMO–OFDM Modem With an SIMD ALU-Based Architecture

In this paper, a single instruction multiple data (SIMD) arithmetic logic unit (ALU)-based architecture is proposed to improve hardware efficiency in a $4 \times 4$ frequency-domain multiple input multiple output–orthogonal frequency division multiplexing modem based on a space-time block code (STBC). The majority of mathematic units in the proposed architecture are centralized so that any mathematic unit can be shared with any algorithm. Six advanced instructions are also defined in the ALU: 1) complex multiplication; 2) complex division; 3) correlation; 4) channel estimation; 5) $4 \times 4$ matrix inversion; and 6) STBC-based decoding. A scheduler is essential to handle all data paths and signaling flows smoothly with the use of an SIMD ALU. As a result, it is relatively easy to reconfigure the proposed design for different specifications. The very large scale integration implementation of this chip, using an in-house 65-nm CMOS process, consumes a total of 1.87 M gates and draws 33.7 mW at a supply voltage of 1 V.

An Optical OFDM Modem with Adaptive Volterra Equalizer

AbstractIt addresses orthogonal frequency division multiplexing (OFDM) transmission over optical links with high spectral efficiency, i.e. by using high-order quadrature amplitude modulation (QAM) schemes as a mapping method prior to the OFDM multicarrier representation. Here we address especially coherent optical OFDM modem in long distance which is affected by nonlinear distortion caused by fiber nonlinearity. Fiber nonlinearity is a majo performance-limiting factor in advanced optical communication systems. We proposed a nonlinear electrical equalization scheme based on the Volterra model. To compare with other popular linear compensation technique such as the least mean square (LMS), simulation results are presented to demonstrate the capability of a Volterra model based electrical equalizer used in a coherent optical orthogonal frequency division multiplexing system. It is shown that the Volterra model based equalizer can significantly reduce nonlinear distortion.

OFDM-Modulated Dynamic Coded Cooperation in Underwater Acoustic Channels

Dynamic coded cooperation (DCC) allows relay diversity without altering the transmission procedure from the source to the destination. In this paper, we propose a practical orthogonal frequency-division multiplexing (OFDM)-modulated DCC scheme for underwater relay networks, where OFDM modulation accommodates multipath fading channels with large delay spread. Two cooperation strategies are studied, where the relay transmits either identical or different OFDM blocks as the source during the cooperation phase. The block-level synchronization between the OFDM blocks from the source and the relay is achieved by a delay control mechanism at the relay, by knowing the distances among the source, relay, and destination. Two OFDM-DCC design examples are presented, one based on nonbinary rate-compatible low-density parity-check (LDPC) codes applied across multiple OFDM blocks, and the other using layered interblock erasure correction and intrablock error-correction coding. In addition to simulation studies, one particular design has been implemented on practical OFDM modems, and tested in a swimming pool and in a recent sea experiment. The proposed OFDM-DCC scheme is particularly appealing to underwater acoustic (UWA) networks where a relay node with abundant resources (e.g., a surface buoy) can enhance communications among underwater nodes without changing their transmission procedure.

Resource Efficient Implementation of Low Power MB-OFDM PHY Baseband Modem With Highly Parallel Architecture

The multi-band orthogonal frequency-division multiplexing modem needs to process large amount of computations in short time for support of high data rates, i.e., up to 480 Mbps. In order to satisfy the performance requirement while reducing power consumption, a multi-way parallel architecture has been proposed. But the use of the high degree parallel architecture would increase chip resource significantly, thus a resource efficient design is essential. In this paper, we introduce several novel optimization techniques for resource efficient implementation of the baseband modem which has highly, i.e., 8-way, parallel architecture, such as new processing structures for a (de)interleaver and a packet synchronizer and algorithm reconstruction for a carrier frequency offset compensator. Also, we describe how to efficiently design several other components. The detailed analysis shows that our optimization technique could reduce the gate count by 27.6% on average, while none of techniques degraded the overall system performance. With 0.18-μm CMOS process, the gate count and power consumption of the entire baseband modem were about 785 kgates and less than 381 mW at 66 MHz clock rate, respectively.

Simplified adaptively modulated optical OFDM modems using subcarrier modulation with added input/output reconfigurability

Three novel designs of adaptively modulated optical orthogonal frequency division multiplexing modems using subcarrier modulation (AMOOFDM-SCM) are proposed, for the first time, each of which requires a single inverse fast Fourier transform/fast Fourier transform (IFFT/FFT) operation. These designs not only significantly simplify the AMOOFDM-SCM modem configurations, but also offer extra unique network features such as input/output reconfigurability. Investigations show that these three modems are capable of supporting more than 60 Gb/s AMOOFDM-SCM signal transmission over 20, 40 and 60 km single mode fibre (SMF)-based intensity modulation and direct detection (IMDD) transmission links without optical amplification and chromatic dispersion compensation.

Vulnerabilities of underwater acoustic networks to denial‐of‐service jamming attacks

AbstractRecent surges in the development of underwater acoustic networks (UANs) have lead to a rapid acceptance of this technology in scientific, commercial, and military applications. However, limited work has been performed on developing secure communication mechanisms and techniques to protect these networks. Security mechanisms are wildly studied in terrestrial networks, and various defense mechanisms have been developed as safeguards. Because of the difference in communication mediums and physical environments, the existing solutions for terrestrial networks cannot be directly applied for UANs. In this paper, we study the effects of denial‐of‐service jamming attacks on UANs using real‐world field tests. We develop our own jammer hardware and signals in order to analyze the characteristics of different jamming attack models on a network. Our tests are performed on multiple commercial brand acoustic modems and an orthogonal frequency division multiplexing modem prototype. We show that UANs can be easily jammed using carefully timed attacks, which are energy efficient. Copyright © 2012 John Wiley & Sons, Ltd.

Modified OFDM Model Based on Sub-Sliding Window Technique

In modern wireless communication systems, Doppler frequency reduction is employed for efficient transmission of data in noisy environments. Achieving a very less bit error rate (BER) has been the major task in the orthogonal frequency division multiplexing (OFDM), which is a popular multicarrier modulation technique in IEEE 802.11a wireless systems and is currently being used in the IEEE 802.16e (WiMax) standard. This paper presents a modified sub-sliding window (SSW) technique that is used to improve the performance of OFDM modem for the physical layer of IEEE 802.11a standard. To carry out the simulations, the bit error rate performance of the modified and standard models is compared under the additive white Gaussian noise (AWGN) and flat fading channel. The simulation results show that the combination of SSW and OFDM technology is superior to the OFDM standard technology.

Improving the chromatic dispersion tolerance in long-haul fibre links using the coherent optical orthogonal frequency division multiplexing

Numerical simulations of the coherent optical orthogonal frequency division multiplexing modems are undertaken to investigate the effect of the adaptive modulation, the number of sub-carriers, the cyclic prefix (CP) length, the clipping ratio, quantisation bit resolution and the sampling speed of analogue-to-digital converters (ADCs) on the chromatic dispersion (CD) of a single mode fibre (SMF) at data rates up to 80 Gbps. The use of a large number of sub-carriers is more effective in combating fibre dispersion than employing a long CP; moreover, the optimum number of sub-carriers in the presence of both SMF non-linearities and CD has been identified. The authors show that using a high bit resolution ADC with a high clipping ratio, the transmission distance can be increased at specific data rates. Furthermore, it is shown that ADCs with a low sampling speed also improve the system tolerance to the fibre CD. In addition, simulation results show that the use of adaptive modulation schemes improves spectrum usage efficiency, thus resulting in higher tolerance to the CD when compared with the case in which identical modulation formats are adopted across all sub-carriers.

Effect of number of sub-carriers, cyclic prefix and analogue to digital converter parameters on coherent optical orthogonal frequency division multiplexing modem's transmission performance

The number of sub-carriers, cyclic prefix (CP), and the characteristics of the analogue to digital converter (ADC), are significant parameters influencing the transmission capacity and distance of an optical orthogonal frequency division multiplexing (OOFDM) modem. Numerical simulations of coherent OOFDM modems are undertaken to investigate the effect of the number of sub-carriers, the CP length and ADC associated parameters such as the sampling speed, clipping ratio and quantisation bit on the system performance over single mode fibre (SMF) links for data rates up to 80 Gb/s. The use of a large number of sub-carriers is more effective in combating fibre dispersion when compared with employing a long CP. Moreover in the presence of fibre non-linearities identifying the optimum number of sub-carriers is crucial factor for the modem performance. For various signal data rates up to 40 Gb/s, a set of data rate and transmission distance-dependent optimum ADC parameters are identified. These parameters give rise to negligible clipping and quantisation noise, moreover ADC sampling speed can increase dispersion tolerance while transmitting over SMF links.

Error Analysis and Verification of an IEEE 802.11 OFDM Modem using Theorem Proving

IEEE 802.11 is a widely used technology which powers many of the digital wireless communication revolutions currently taking place. It uses OFDM (Orthogonal Frequency Division Multiplexing) in its physical layer which is an efficient way to deal with multipath, good for relatively slow time-varying channels, and robust against narrowband interference. In this paper, we formally specify and verify an implementation of the IEEE 802.11 standard physical layer based OFDM modem using the HOL (Higher Order Logic) theorem prover. The versatile expressive power of HOL helped us model the original design at all abstraction levels starting from a floating-point model to the fixed-point design and then synthesized and implemented in FPGA technology. We have been able to find a bug in one of the blocks of the design that is responsible for modulation which implementation diverts from the constellation provided in the IEEE standard specification. The paper also derives new expressions for the rounding error accumulated during ideal real to floating-point and fixed-point transitions at the algorithmic level and performs a formal error analysis for the OFDM modem in HOL.

Analysis of phase noise effects in OFDM modems

The effects of the phase noise (PN) on orthogonal frequency-division multiplex modems are evaluated. Three receivers are studied: a coherent receiver, a common phase error correction receiver (which is a receiver specially designed to combat PN) and a differential receiver. The impact of the PN on the decision signal-to-noise ratio (SNR) of each of these receivers is computed as a function of the PN spectrum. The resulting formulas are extremely simple. The theory is applicable to a wide range of PN models, and unifies and extends previous results on the topic. The conditions under which the decision SNR yields correct symbol error rate predictions are discussed. Simulations are reported that confirm the results.

Analysis of the Effect of the I/Q Base-Band Filter Mismatch in an OFDM Modem

In digital communication modems in which a very high rate system clock is used, it is necessary to use analog base-band shaping filters in the inphase (I) and quadrature (Q) paths of the modulator. However, this type of implementation inherently produces a mismatch of the I and Q paths. In the present paper, results of the analysis of the transmitter (TX) I/Q mismatch in an Orthogonal Frequency Division Multiplexing (OFDM) system with Differential Coherent Quadrature Phase Shift Keying (DQPSK) modulation is presented. Theoretical analysis shows that the Signal-to-Noise (SNR) degradation due to the I/Q mismatch can be represented by a mismatch transfer function on the basis of which one can compute the maximum affordable amplitude and phase mismatch of the TX filters transfer functions.

デジタル放送を支える伝送技術 ＤＡＰＳＫ‐ＯＦＤＭ変調方式を用いた高速データ伝送の実験的検討

Digital transmission signals are distorted by multi-path fading in a mobile radio channel. This is a severe problem, especially when a digital field pick-up unit transmits high-bit-rate data such as HDTV digital signals. The OFDM (orthogonal-frequency-division-multiplexing) modulation scheme mitigates the adverse effects of multi-path fading by distributing the transmission data into a large number of carriers with low-bit-rate modulation. To reduce the degradation due to multi-path fading in high-bit-rate transmission, we have developed an experimental OFDM modem that uses multi-mode modulation : 16/32 DAPSK (differential amplitude phase shift keying). Transmission experiments showed that the a 16/32 DAPSK-OFDM modem can transmit data at 23.7 Mbps by 16 DAPSK modulation and at 29.7 Mbps by 32 DAPSK modulation within a bandwidth of 13.5 MHz in a frequency-selective Rayleigh fading channel.

ディジタル放送 ＤＱＰＳＫ‐ＯＦＤＭ変調方式によるＵＨＦ帯ＦＰＵの移動伝送特性

The OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is robust under multipath fading conditions. An experimental OFDM modem using the Differential Quadrature Phase Shift Keying (DQPSK) modulation scheme for each carrier has been developed to enable stable transmission of high-rate digital sources from moving vehicles. As one of the promising applications of the DQPSK-OFDM modulation scheme, a digital Field Pick-up Unit (FPU) is expected to be efficient in the case of mobile transmission. This paper describes the transmission characteristics of the DQPSK-OFDM modem in an urban area which transmits digital sources from vehicles in the UHF 800-MHz band, and also discusses the error protection scheme, the guard interval length, and the carrier spacing of the DQPSK-OFDM modem for mobile transmission.

Experimental and Simulation Results on an OFDM Modem for TV Broadcasters

In the application of the orthogonal frequency division multiplexing (OFDM) systems to digital television broadcasting, it is desirable to increase the bit rate in the same bandwidth without seriously impairing transmission quality. For this purpose, multilevel OFDM systems are suitable. This paper describes a prototype multilevel OFDM modem developed for experimental transmission of digital terrestrial broadcasting signals. The reference data that provide phase and amplitude references of each carrier are sent periodically and are used in the demodulation. The basic concept for the design of the modem is explained and the related simulation results are presented. Preliminary experimental results are also shown and the transmission characteristics of multilevel OFDM are discussed.

Digital outside-broadcasting-link using OFDM modulation scheme

A study on digital broadcasting systems has been conducted for the multimedia era. Regarding program production, a digital outside-broadcasting-link (OBL), in particular, from mobile source is required to gather digital video and audio sources in severe multipath environments. The orthogonal frequency division multiplexing (OFDM) modulation scheme is robust under multipath fading conditions. A digital OFDM modem for OBL in the 800 MHz band has been developed to enable stable transmission of digital sources from moving vehicles. In the experimental OFDM-OBL, the compressed digital video and audio data are distributed to 572 carriers in the 9 MHz bandwidth, using differential quadrature phase shift keying (DQPSK) modulation for each carrier. In computer simulation and indoor/outdoor experiments, it is confirmed that digital OFDM-OBL can stably transmit signals, even under severe multipath fading conditions, while pictures transmitted with conventional FM-OBL are seriously degraded. This paper describes the features of the OFDM modem for the digital OBL which transmits program sources from moving vehicles, and also discusses aspects of gathering digital program sources.